In a wide variety of devices in various fields, an object is illuminated with a high-intensity illumination in a long, narrow illumination pattern, i.e., an "elongate" is illumination pattern, and the light returned from the illuminated portion of the object is detected. For example, in a scanner or photocopier, the object illuminated is a document, and the light returned from the illuminated portion of the document is detected by focusing this light on a CCD in a scanner or on a light-sensitive drum in a photocopier. Other applications of object imaging include surface inspection devices, such as those used in the electronics industry. Object imaging also includes applications in which the total flux of the light reflected by the illuminated portion of the object is detected and no image formed at the detector. To simplify the following description, the word "imaging" will be taken to include these applications in which the total light flux is detected and no image is actually formed at the detector.
Conventional scanners and photocopiers often utilize linear extended light sources, such as fluorescent lamps. The light emitted by the lamp must be transferred to the object so that it provides a high illumination intensity within the elongate illumination pattern. The illumination intensity must be sufficiently high for the light returned from the illuminated part of the object to have sufficient intensity to allow the returned light to be detected reliably. For example, in a scanner, insufficient light intensity at the document results in the detected image having an unacceptable signal-to-noise ratio.
Since the light sources of conventional scanning and photocopying systems are lambertian and not directional, much of the light generated by the light source does not impinge the region of the surface of the object that is to be imaged. A high-power illumination assembly is often used to ensure that the illumination intensity at the object is adequate. Since the cost of the power supply of the illumination assembly is a significant part of the overall cost of the system, the cost of illuminating the object with low efficiency translates directly to cost inefficiency.
The inefficiency of an illumination assembly for imaging an object can be reduced by providing reflective surfaces near the light source. U.S. Pat. No. 4,699,497 to Hilton et al. describes an illumination assembly for a document scan system that includes a long, cylindrical lamp partially enclosed by a pair of reflectors that constitute two arcs of a single ellipse. The lamp emits light through 360.degree.. The patent states that the prime requirement for placing the lamp relative to the elliptically-shaped reflectors is to locate the center of the lamp at one of the foci of the ellipse of which the reflectors constitute arcs. The reflectors concentrate and redirect the light emitted by the lamp, and so enhance the efficiency of the illumination assembly.
Another system that uses reflective surfaces to define an illumination field for an optical scanner is described in U.S. Pat. No. 5,058,982 to Katzir. The Katzir optical scanner is a system for inspection of printed circuit boards, wafers and the like. Each of two elongate light sources has a reflective focusing member and a lenticular lens sheet that spreads the light from its light source over the associated reflector focusing member. The illumination system also includes a third elongate light source that is used to produce a brightfield illumination.
While the prior art imaging systems of Hilton et al. and Katzir improve the efficiency of illumination, further performance enhancements are desired.
Other illumination assemblies place a fluorescent aperture lamp close to the surface of the object to be imaged. The aperture lamp includes an internal reflector, and emits light from an elongate aperture disposed along the length of the lamp. The light emitted from the aperture has a substantially higher intensity than the light emitted by an equivalent non-aperture fluorescent lamp. However, in the plane perpendicular to the length of the lamp, an aperture lamp emits light through an angle of 180 degrees. Thus, such lamps provide an elongate illumination pattern on a nearby object at only a low efficiency because most of the light emitted by the lamp falls outside the illumination pattern. Increasing the efficiency of such lamps requires as much of the light emitted in the 180.degree. angle as possible be captured and concentrated to form the illumination pattern on the object. Conventional reflectors or lenses capable of performing this task are expensive and bulky.
Compound elliptical concentrators ("CECs") are known in the art for illuminating distant objects using a plane source of radiation. A device for spreading infra-red radiation from a plane source symmetrically over a distant extended target is described in U.S. Pat. No. 4,922,107 of Rabl et al. The CEC collects all the radiation emitted through a 180.degree. angle by the plane infrared source.
What is needed is a way to adapt a CEC to illuminate a narrow strip of a nearby object with the light from an aperture lamp in a manner that enables the light returned from the illuminated portion of the object to be coupled to a suitable detector.